Capillary adhesion of stick insects

Scientific progress within the last few decades has revealed the functional morphology of an insect's sticky footpads -- a soft, sponge-like pad that secretes a thin liquid film. However, the physico-chemical mechanisms underlying their adhesion remain elusive. Here, we explore these underlying mechanisms by simultaneously measuring adhesive force and contact geometry of the adhesive footpads of live, tethered Indian stick insects, \textit{Carausius morosus}, spanning more than two orders of magnitude in body mass. We find that the adhesive force we measure is similar to previous measurements that use a centrifuge. Our measurements afford use the opportunity to directly probe the adhesive stress \textit{in vivo}, and use existing theory on capillary adhesion to predict the surface tension of the secreted liquid and compare it to previous assumptions. From our predictions, we find that the surface tension required to generate the adhesive stresses we observed ranges between 0.68 mN/m and 12 mN/m. The low surface tension of the liquid would enhance the wetting of the stick insect's footpads and promote their ability to conform to various substrates. Our insights may inform the biomimetic design of capillary-based, reversible adhesives and motivate future studies on the capillary properties of the secreted liquid.